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Journal of Materials Science

, Volume 53, Issue 12, pp 9046–9063 | Cite as

Integration of NiWO4 and Fe3O4 with graphitic carbon nitride to fabricate novel magnetically recoverable visible-light-driven photocatalysts

  • Mitra Mousavi
  • Aziz Habibi-Yangjeh
Composites

Abstract

Novel magnetically recoverable g-C3N4/Fe3O4/NiWO4 (gCN/M/NiWO4) nanocomposites, with superior visible-light photocatalytic performance, were successfully fabricated by a refluxing calcination method. These hybrid photocatalysts were characterized fairly in terms of the structure, composition, morphology, electronic, textural, thermal, and magnetic properties using XRD, EDX, SEM, TEM, HRTEM, FT-IR, UV–vis DRS, PL, N2 adsorption–desorption, TG, and VSM analyses. Also, the degradation intermediates were identified using gas chromatography–mass spectroscopy. These photocatalysts displayed excellent photocatalytic performance under visible light for degradations of RhB, MB, MO, fuchsine, and phenol pollutants, and they can be recycled by magnetic separation without major loss of activity. The highest photocatalytic efficiency was observed when the sample refluxed for 60 min and calcined at 450 °C for 3 h with 30 wt% NiWO4 content. Activity of this photocatalyst is greater than the pristine gCN by a factor of almost 12, 30, 52, 100, and 6 toward degradations of RhB, MB, MO, fuchsine, and phenol, respectively. Finally, the proposed mechanism for the superior performance of gCN/M/NiWO4 hybrid photocatalysts was discussed.

Introduction

As known, utilization of the solar energy, as a powerful renewable energy, is regarded as the most effective strategy to address energy and pollution crises [1, 2]. Semiconductor-based photocatalysis has attracted great attention from researchers for production of green energy and degradation of wide variety of pollutants in our environment [3, 4, 5, 6, 7]. In these processes, carbon dioxide is converted to fuels and value-added compounds such as methane, methanol, formic acid, and formaldehyde [5]. Additionally, hydrogen fuel is produced through photocatalytic splitting of water [6, 7, 8, 9, 10, 11]. Despite more than three decades of efforts devoted toward preparation and application of more efficient visible-light-driven photocatalysts to convert the solar energy to renewable fuels and environmental remediation, there are big challenges in this field. Hence, researchers have focused on fabrication of more efficient and economically effective photocatalysts using different organic/inorganic semiconductors [12, 13]. To achieve these desired properties, the prepared photocatalysts should have high visible-light response, minimal recombination for the photoproduced e/h+ pairs, good recyclability, and stability. The usual photocatalysts such as TiO2, ZnO, SnO2, and ZnS have drawbacks of large band gap and fast recombination of the charge carriers [14, 15, 16, 17]. More recently, graphitic carbon nitride (g-C3N4, denoted as gCN) has gained more attention from the research community due to its appealing electronic and stability properties [18, 19, 20]. This polymeric semiconductor with narrow band gap (~ 2.70 eV) has displayed wide applications in photocatalytic processes under visible-light illumination [21]. Nonetheless, poor absorption of visible light, low quantum efficiency caused by rapid recombination of the e/h+ pairs, and nonmagnetic properties limit its large-scale application. To address these problems, several strategies have been developed such as doping of metals and/or nonmetals, fabrication of porous gCN, integration with other narrow band gap/wide band gap semiconductors, and decoration of magnetic nanoparticles over gCN sheets [22, 23, 24, 25, 26, 27, 28, 29].

In recent years, metal tungstates (MWO4, M is usually Cu2+, Co2+, Ni2+, and Mn2+) have gained much attention in different disciplines such as photocatalytic processes, optical fibers, pigments, supercapacitors, and gas sensors due to their fascinating optical and magnetic properties [30, 31, 32, 33, 34, 35]. Among these metal tungstates, NiWO4 is small-band gap semiconductor (Eg = 2.2 eV) with low price and environmentally green benign [31]. Hence, this semiconductor can help to solve poor visible-light absorption and rapid recombination of e/h+ pairs through proper band alignments. Finally, to overcome recoverability of gCN after the photocatalytic process, decoration of Fe3O4, as a usual magnetic material, is attractive option. Nanoparticles of Fe3O4 have attracted much more attention from research communities to apply in different disciplines, owing to its appealing properties including availability of facile synthesis methods, biocompatibility, high saturation magnetization, and high stability [36, 37, 38, 39, 40, 41].

In this study, novel hybrid g-C3N4/Fe3O4/NiWO4 (gCN/M/NiWO4) photocatalysts were synthesized by integration of Fe3O4 and NiWO4 with gCN using refluxing calcination method. Activity of the hybrid photocatalysts was explored by degradations of rhodamine B (RhB), methylene blue (MB), methyl orange (MO), fuchsine, and phenol under visible-light illumination. Different characterization techniques of XRD, EDX, SEM, TEM, HRTEM, FT-IR, UV–vis DRS, PL, N2 adsorption–desorption, TG, and VSM were applied to disclose phase structure, elemental composition, morphology, optical, textural, thermal, and magnetic properties in detail. We investigated the intermediates formed during photodegradation of RhB by gas chromatography–mass spectroscopy (GC–MS). After investigation about the effect of various operational factors on the photocatalytic activity, a possible degradation mechanism for the excellent photocatalytic performance of the gCN/M/NiWO4 nanocomposites was proposed.

Experimental

Materials

All chemicals with analytical grade were used as received from Loba Chemie and Merck companies.

Instruments

The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). Morphology of the samples was studied by LEO 1430VP SEM. Purity of the products was obtained by EDX on the same SEM instrument. The TEM investigations were performed by a Philips CM30 instrument with an acceleration voltage of 150 kV. High-resolution TEM (HRTEM) image was used to further analyze the morphology of the products on a JEOL-JEM-2100F transmission electron microscope with an accelerating voltage of 200 kV. The UV–vis DRS were recorded by a Scinco 4100 apparatus. The FT-IR spectra were obtained by a Perkin Elmer Spectrum RX I apparatus. The PL spectra of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer. The UV–vis spectra were plotted by a Cecile 9000 spectrophotometer. The GC–MS analysis was carried out by Agilent 7890B Series GC and Agilent 5977A Series MSD with column HP-5MS size 30 m × 0.25 mm. Magnetic properties of the samples were obtained using a VSM instrument (Meghnatis Kavir Kashan Co., Iran). The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz).

Preparation section

Pure gCN was synthesized by thermal polycondensation of melamine up to 520 °C in a muffle furnace [18, 19, 20, 21]. The g-C3N4/Fe3O4 (2:1), denoted as gCN/M nanocomposite, in which 2:1 is weight ratio of g-C3N4 to Fe3O4, was prepared by in situ co-precipitation of Fe2+ and Fe3+ ions adsorbed over the dispersed gCN [42]. These ions have strong interactions with the nitrogen atoms in the cavity of the gCN host provided by nitrogen atoms. In detail, the cations of early transition metals (i.e., Fe) are almost symmetrically embedded in the gCN cavity. Size of these cations is responsible for this affinity, as confirmed in the literature [43]. The gCN/M/NiWO4 (30%) nanocomposite was fabricated by refluxing calcination method. In a typical procedure, 0.35 g of gCN/M nanocomposite was dispersed in water (150 mL) by ultrasonic irradiation for 5 min. After that, 0.142 g of nickel nitrate was added to the suspension and the stirring continued for 60 min. Then, sodium tungstate (0.161 g in 20 mL of water) was dropwise added to the suspension and refluxed for 60 min. The resultant light brown suspension was then centrifuged to get the precipitate out and washed with water and ethanol. Finally, the dried sample at 60 °C was calcined at 450 °C for 3 h in air (Scheme 1).
Scheme 1

Preparation procedure for the gCN/M/NiWO4 nanocomposites

Photocatalysis section

Photocatalytic activity of the fabricated samples was explored at controlled temperature of 25 °C under visible-light illumination provided by an LED lamp with 50 W. Concentrations of RhB, MB, and MO were 1 × 10−5 M, concentration of fuchsine was 0.77 × 10−5 M, and concentration of phenol was 5 × 10−5 M. Other conditions were described in detail in our previous work [42].

Results and discussion

The phase structures of gCN, gCN/M, and gCN/M/NiWO4 photocatalysts were explored, and the XRD patterns are displayed in Fig. 1. In the gCN, the interlayer stacking and in-planar peaks are observed at 27.6° and 12.9°, respectively (JCPDS No. 87-1526) [20]. In the gCN/M photocatalyst, besides the diffraction peaks of gCN, six diffraction peaks in the range of 2θ = 30.6° − 63.2° are observed, which are ascribed to (220), (311), (400), (422), (511), and (440) planes of Fe3O4 (JCPDS No. 89-0691) [42, 44]. For the XRD patterns of gCN/M/NiWO4 nanocomposites, along with the characteristic peaks of g-C3N4 and Fe3O4, the characteristic peaks of NiWO4 are visualized, which are in good agreement with wolframite phase structure (JCPDS-15-0755) [31]. Additionally, for the gCN/M/NiWO4 nanocomposites, the diffraction peak of g-C3N4 does not shift after integration with Fe3O4 and NiWO4 counterparts, suggesting that the introduced materials do not insert into the lattice of gCN and they are simply decorated over its sheets.
Figure 1

XRD patterns for the gCN, gCN/M, and gCN/M/NiWO4 nanocomposites with different weight percentages of NiWO4

The EDX spectra of the as-prepared photocatalysts were used to show purity and estimate elemental composition, as shown in Fig. 2a. All of the elements in the spectra are clearly observed at their known energies. The corresponding peaks of C and N for the gCN sample, C, N, Fe, and O peaks for the gCN/M sample are clearly observed. In addition, elemental analysis of the gCN/M/NiWO4 (30%) nanocomposite confirms the presence of C, N, Fe, O, Ni, and W elements in this hybrid photocatalyst, confirming successful fabrication of the ternary photocatalyst. Finally, elemental homogeneity in the gCN/M/NiWO4 (30%) nanocomposite was further examined by EDX mapping, as displayed in Fig. 2b–h. These images indicate homogeneous distribution of the elements, establishing homogeneous distribution of Fe3O4 and NiWO4 over the sheets of gCN. Weight percentages of gCN, Fe3O4, and NiWO4 in the gCN/M/NiWO4 (30%) nanocomposite were calculated to be 46.9, 23.4, and 29.7 wt%, respectively, which are similar with the preparation contents. Also, weight percentages of different elements are 18.2, 28.4, 16.9, 12.7, 5.77, and 17.9 for C, N, Fe, O, Ni, and W, respectively.
Figure 2

a EDX spectra for the gCN, gCN/M, and gCN/M/NiWO4 (30%) samples. bh EDX mapping for the gCN/M/NiWO4 (30%) nanocomposite

To investigate morphologies, SEM, TEM, and HRTEM images of the gCN/M/NiWO4 (30%) nanocomposite were provided and they are displayed in Fig. 3a–c. It is obvious that Fe3O4 and NiWO4 have decorated over surface of gCN. The black particles in the TEM image are assigned to Fe3O4 and NiWO4 counterparts, and the gray-colored lamellar material is ascribed to gCN. Although before the TEM analysis, the sample was dispersed by strong ultrasonic irradiation, the decorated Fe3O4 and NiWO4 particles did not peel off from gCN surface, implying that the interaction between Fe3O4 and NiWO4 with gCN is considerably strong. This interaction could be beneficial for separation of the e/h+ pairs during the photocatalytic processes. From HRTEM image (Fig. 3c), three different kinds of lattice fringes are clearly observed. The fringe with d = 0.326 nm matches with the (002) plane of gCN [45], another with d = 0.24 nm is corresponded to the (311) plane of Fe3O4 [46], and the last one with d = 0.28 nm is attributed to the (111) plane of NiWO4 [31].
Figure 3

a SEM, b TEM, and c HRTEM images of the gCN/M/NiWO4 (30%) nanocomposite

The presence of various components in the hybrid photocatalyst was further explored by FT-IR spectroscopy, as represented in Fig. 4a. The spectrum of pure gCN shows the skeletal vibrations for aromatic heterocycles at the range of 1200–1600 cm−1 and the breathing vibration for its units at 810 cm−1. A broad band in the range of 3000–3500 cm−1 is attributed to symmetric and asymmetric vibrations of NH and NH2 of uncondensed amino groups and adsorbed water [23]. In the gCN/M nanocomposite, the characteristic vibration peak of Fe3O4 is observed at 550 cm−1, corresponding to the Fe–O bond [47]. Furthermore, the vibration bands at 656 and 822 cm−1 in the spectrum of gCN/M/NiWO4 (30%) nanocomposite are related to the W–O and O–W–O stretching vibration modes, respectively [30, 31]. Hence, all of the main characteristic vibration peaks of gCN, Fe3O4, and NiWO4 counterparts are appeared in the spectrum of gCN/M/NiWO4 (30%) nanocomposite.
Figure 4

a FT-IR spectra for the gCN, gCN/M, and gCN/M/NiWO4 (30%) samples. b UV–vis DRS for the gCN, gCN/M, and gCN/M/NiWO4 nanocomposites with different weight percentages of NiWO4

To gain information about optical properties of the synthesized photocatalysts, UV–vis DRS spectra of the gCN, gCN/M, and gCN/M/NiWO4 nanocomposites were investigated. As displayed in Fig. 4b, the gCN powder has strong absorption in UV region and poor absorption in visible region. Obviously, the gCN/M nanocomposite shows more intensive absorption in visible region in comparison with the gCN. Interestingly, absorption intensity of the gCN/M/NiWO4 nanocomposites remarkably increased in visible region and the absorption enhances smoothly with increasing NiWO4 content, implying that with introduction of NiWO4 in the gCN/M nanocomposite, photocatalytic performance could be favored under visible-light illumination.

To explore thermostability and estimate decoration percent of Fe3O4 and NiWO4 over gCN, TG analysis was carried out and the results are observed in Fig. 5a. The samples do not have considerable weight loss up to 400 °C. These small weight losses are related to evaporation of adsorbed solvent molecules remained from purification step. For the gCN powder, the pronounced weight loss starts from 490 °C and continues up to 670 °C, which is ascribed to the decomposition of this polymeric semiconductor [18, 19, 20]. Obviously, the gCN/M and gCN/M/NiWO4 nanocomposites begin to decompose at lower temperatures than the pristine gCN, which implies that thermal stability of gCN decreases with decoration of other materials over it [48, 49]. Additionally, contents of gCN in the nanocomposites were estimated from the remained weights after complete decomposition of the organic counterpart at 700 °C. The gCN contents in gCN/M and gCN/M/NiWO4 nanocomposites with 10, 20, 30, and 40% of NiWO4 were determined to be 73.3, 62.4, 54.7, 47.8, and 42.3 wt%, respectively.
Figure 5

a TG plots for the gCN, gCN/M, gCN/M/NiWO4 nanocomposites with different weight percentages of NiWO4. b VSM curves for the Fe3O4 and gCN/M/NiWO4 (30%) samples

Figure 5b shows magnetization curves for the Fe3O4 and gCN/M/NiWO4 (30%) samples, which are measured at room temperature by sweeping the applied magnetic field from − 8 to 8 kOe. It is noteworthy that the hysteresis loops are S-like, indicating that there is not remaining magnetization after removing external magnetic field. Hence, these samples display superparamagnetic behavior. The saturation magnetization is 55.5 emu g−1 for the Fe3O4 nanoparticles and 6 emu g−1 for the gCN/M/NiWO4 (30%) nanocomposite. Compared with the pure Fe3O4 nanoparticles, this decrease of the saturation magnetization is associated with the existence of nonmagnetic gCN and NiWO4 in the structure of the hybrid photocatalyst. Magnetic separability of the gCN/M/NiWO4 (30%) nanocomposite was tested in an aqueous suspension of the nanocomposite by placing a magnet next to the bottle. The brown suspension was rapidly attracted toward the magnet only within 15 s (in inset of Fig. 5b), demonstrating that the ternary photocatalyst has an excellent magnetic recoverability.

Figure 6 displays photocatalytic performance of the fabricated photocatalysts under visible light. It is obvious that under the selected light source, photolysis of RhB without using any photocatalyst is low. When the gCN and gCN/M photocatalysts are employed in the system, the degradation efficiencies of RhB are enhanced. In addition, it should be noted that after decoration of NiWO4 over the gCN/Fe3O nanocomposite, a remarkable enhancement for degradation of RhB was observed. The removal percentages were reached to 61, 88, 100, and 83% over the gCN/M/NiWO4 (10%), gCN/M/NiWO4 (20%), gCN/M/NiWO4 (30%), and gCN/M/NiWO4 (40%) hybrid photocatalysts, respectively. Therefore, the gCN/M/NiWO4 (30%) nanocomposite displayed the superior activity among the ternary nanocomposites.
Figure 6

Photodegradation of RhB over the gCN, gCN/M, and gCN/M/NiWO4 nanocomposites with different weight percentages of NiWO4

Figure 7 displays time courses for degradation of RhB over the gCN, gCN/M, and gCN/M/NiWO4 (30%) photocatalysts under the light illumination. As can be seen, the main absorption peaks of RhB at 553 nm decrease with extension of the irradiation time and about 38.7 and 48% of RhB are degraded within 240 min of the light irradiation over the gCN and gCN/M samples, respectively. While, the absorption peak is completely disappeared at the same time of the light irradiation over the gCN/M/NiWO4 (30%) photocatalyst, which implies complete degradation of RhB. This stepwise degradation without any shift in the main peak of RhB is ascribed to its photodegradation through ring-opening mechanism [50, 51].
Figure 7

UV–vis spectra for degradation of RhB under visible-light irradiation over the a gCN, b gCN/M, and c gCN/M/NiWO4 (30%) nanocomposites

In this part, GC–MS analysis was also used to detect the intermediates for degradation of RhB over the gCN/M/NiWO4 (30%) photocatalyst illuminated by visible light to provide an additional evidence for the photodegradation reaction. The GC–MS results for the final photodegradation reaction of RhB are shown in Fig. 8a, b. The predominant peak, marked by star, was observed for this system. This peak with the retention time of 5.82 min (Fig. 8a) has been assigned to benzoic acid by comparing the molecular ion and mass fragmentation pattern with GC–MS library (Fig. 8b). Some publications have investigated photodegradation pathways for RhB in the presence of different photocatalysts [52, 53]. Although a series of intermediates have been found during the degradation reaction of RhB, benzoic acid is widely observed intermediate, due to its substantial stability [52, 53].
Figure 8

a GC chromatogram analysis for photodegradation of RhB over the gCN/M/NiWO4 (30%) nanocomposite. b Mass spectrum of the intermediate

Kinetic behavior of the degradation reaction over the photocatalysts was studied by pseudo-first-order equation, and the calculated rate constants are displayed in Fig. 9a. Obviously, when NiWO4 content over the gCN/M was below 30 wt%, the photocatalytic activity was enhanced with the increase of NiWO4 content. However, when the NiWO4 content exceeded 30 wt%, the photocatalytic activity decreased. Hence, the excess of NiWO4 may compete with gCN in absorption of the irradiated light, leading to shielding of gCN from production of the charge carriers. Consequently, an optimum amount of NiWO4 should be decorated over the gCN to help it in capturing photogenerated electrons to suppress separation of the e/h+ pairs. The rate constant for RhB degradation by the gCN/M/NiWO4 (30%) nanocomposite (146.1 × 10−4 min−1) is about 12 times larger than that of the gCN (12.17 × 10−4 min−1) and 11.2 times higher than that of the gCN/M nanocomposite (13.1 × 10−4 min−1). These results indicate that NiWO4 has more effective role in enhancing photocatalytic performance of gCN.
Figure 9

a The degradation rate constants of RhB over the gCN, gCN/M, and gCN/M/NiWO4 nanocomposites with different weight percentages of NiWO4. b PL spectra for the gCN, gCN/M, and gCN/M/NiWO4 nanocomposites with different weight percentages of NiWO4 samples. c Nitrogen adsorption–desorption data for the gCN, gCN/M, and gCN/M/NiWO4 (30%) nanocomposite

As known, PL analysis is an efficient tool commonly employed to observe separation of e/h+ pairs during photocatalytic reactions. For this purpose, Fig. 9b displays the PL spectra of the gCN/M/NiWO4 nanocomposites along with those for the gCN and gCN/M samples, which are excited by 350 nm and the emission spectra are displayed around 440 nm. As shown, the PL intensity in the gCN/M/NiWO4 (30%) nanocomposite is lower than that of the gCN, gCN/M, and other gCN/M/NiWO4 nanocomposites. Therefore, it is concluded that recombination of the charge carriers in the gCN/M/NiWO4 (30%) nanocomposites is remarkably decreased. It means that in the presence of gCN/M/NiWO4 (30%) nanocomposite, the charge carriers can produce much more reactive species, leading to superior photocatalytic performance. As a consequence, the superior activity of the ternary nanocomposite is attributed to the enhanced visible-light harvesting capacity and effective separation of the e/h+ pairs owing to the proper band alignment between constituents of the nanocomposite.

Nitrogen adsorption–desorption isotherms for the gCN, gCN/M, and gCN/M/NiWO4 (30%) photocatalysts were used to measure their textural properties, as the results are displayed in Fig. 9c and Table 1. The isotherms for all samples are of type IV with hysteresis loops, presenting mesoporous structure [54]. According to the results in Fig. 8c, the measured BET specific surface areas for the gCN, gCN/M, and gCN/M/NiWO4 (30%) photocatalysts are 14.5, 29.4, and 38.50 m2 g−1, respectively. The high specific surface area of the gCN/M/NiWO4 (30%) nanocomposite provides more reaction sites than that of the gCN and gCN/M photocatalysts, resulting in enhanced photocatalytic performance.
Table 1

Textural properties of the gCN, gCN/M, and gCN/M/NiWO4 (30%) nanocomposite

Photocatalyst

Surface area (m2 g−1)

Mean pore diameter (nm)

Total pore volume (cm3 g−1)

gCN

14.6

14.22

0.050

gCN/M

29.4

21.59

0.159

gCN/M/NiWO4

38.5

15.44

0.149

Using the structural characterizations and photocatalytic performance, a possible mechanism for the superior photocatalytic activity of gCN/M/NiWO4 nanocomposites was proposed and is illustrated in Fig. 10. As known, driving force for charge transfers and consequently increase of the photocatalytic activity originate in the matched band potentials of the hybridized semiconductors. As confirmed by PL spectra, due to strong interaction between gCN, Fe3O4, and NiWO4 counterparts of the gCN/M/NiWO4 (30%) nanocomposite, separation efficiency of the e/h+ pairs was considerably increased. The energy gap (Eg) values of gCN and NiWO4 are 2.7 and 2.2 eV, respectively. Under the visible light, gCN and NiWO4 produce e/h+ pairs, because both of them are narrow band gap semiconductors [26, 31]. The zero point potentials for conduction band (CB) and valence band (VB) of gCN are − 1.13 and + 1.57 eV, respectively. Also the CB and VB potentials of NiWO4 are + 0.68 and 2.88 eV, respectively [30]. Hence, the CB potential of gCN is more negative than that of NiWO4. Therefore, the photogenerated electrons of gCN are directly migrated into the CB of NiWO4 through the heterojunction interface formed between these semiconductors. Meanwhile, the photoinduced electrons in the CB of gCN and NiWO4 react with oxygen molecules after diffusion to surface of the nanocomposite to form OH· radicals in successive reduction reactions because the CB potentials of gCN and NiWO4 are more negative than that of the reduction potential of oxygen to hydrogen peroxide (O2 → H2O2 → OH·, E0(O2/H2O2) = + 0.682 eV/NHE) [27]. The produced hydroxyl radicals, with the reduction potential of + 2.80 eV/NHE, have great oxidation power to decompose pollutants [23]. Meanwhile, some of the photogenerated electrons on the CB of gCN can react with O2 to form · O 2 , because the CB potential of gCN is more negative than the potential of O2/·O 2 (E0(O2/ · O 2 ) = − 0.33 eV/NHE) [27]. As depicted in Fig. 10, the excited holes produced over NiWO4 are injected into the VB of gCN. The photoinduced holes over the VB of gCN can oxidize the adsorbed pollutants. Moreover, nanoparticles of Fe3O4 can contribute in separation of the e/h+ pairs. The CB potential of Fe3O4 is positive than that of gCN [55]. Thus, these electrons can easily migrate to the CB of Fe3O4. The injected electrons can react with Fe3+ ions of Fe3O4 to produce Fe2+ ions [56]. Afterward, the formed Fe2+ ions react with O2 to produce Fe3+ ions and.O2 species.
Figure 10

A plausible diagram for the separation of electron–hole pairs in the gCN/M/NiWO4 nanocomposites

In this step, the reactive species capturing experiments were carried out to disclose the role of each photoproduced species during the degradation reaction over the gCN/M/NiWO4 (30%) nanocomposite. The well-known species generally are superoxide anion radicals (·O2), hydroxyl radicals (·OH), and photogenerated holes (h+), which mainly participate in the photocatalytic reactions. Therefore, a series of trapping experiments were performed in the presence of 2-propanol (2-PrOH, quencher of ·OH), benzoquinone (BQ, quencher of ·O2), and ammonium oxalate (AO, quencher of h+) over the gCN/M/NiWO4 (30%) nanocomposite. As displayed in Fig. 11, the degradation rate constant decreased to 95.9 × 10−4 and 113 × 10−4 min−1 in the presence of 2-PrOH and AO scavengers, respectively. These results imply that h+ and ·OH do not have remarkable effect on the degradation reaction of RhB. On the contrary, by introducing BQ, the degradation rate constant reached to 1.76 × 10−4 min−1, which indicates that superoxide anion radicals are the main species in this reaction.
Figure 11

Degradation rate constants of RhB over the gCN/M/NiWO4 (30%) nanocomposite in the presence of various scavengers

On the other hand, the time required for preparation of the photocatalyst was investigated by fabrication of the ternary nanocomposite in different refluxing times of 30, 60, 120, and 240 min and the degradation reaction was studied over these samples, as the results are displayed in Fig. 12a. It is evident that the degradation efficiency firstly increases and then decreases with prolonging refluxing time to 240 min. To gain insight about these changes, morphology and charge separation efficiency data were collected by SEM and PL analyses. As shown in Fig. 12b, c, by prolonging the preparation time, aggregation of the decorated particles significantly increases. Furthermore, Fig. 12d illustrates that intensity of the PL spectrum of the nanocomposite prepared within 240 min is higher than that of the sample prepared in 60 min. These results prove that with further increasing the preparation time, suppression of the e/h+ pairs from recombination is decreased, leading to the decreased photocatalytic activity.
Figure 12

a The degradation rate constants of RhB over the gCN/M/NiWO4 (30%) nanocomposite prepared with different refluxing times. b, c SEM images, and d PL spectra for the gCN/M/NiWO4 (30%) nanocomposite refluxed for 60 and 240 min

As mentioned in the “Experimental” section, the gCN/M/NiWO4 (30%) nanocomposite prepared by refluxing method followed a calcination step. Thus, to select the proper calcination temperature, photocatalytic activity of the gCN/M/NiWO4 (30%) nanocomposite was investigated over the samples calcined at different temperatures, as displayed in Fig. 13a. The results show that the highest activity is observed for the sample calcined at 450 °C and by more increase of this temperature, the degradation efficiency decreases. As shown in Fig. 13b, by increasing the calcination temperature, separation of the e/h+ pairs through transfer of the electrons from the CB of gCN to that of the Fe3O4 and NiWO4 species diminished, resulting in decrease of the activity. On the basis of these results, 450 °C was selected as the optimal calcination temperature in fabrication of the nanocomposites.
Figure 13

a The degradation rate constants of RhB over the gCN/M/NiWO4 (30%) nanocomposite calcined at different calcination temperatures. b PL spectra for the gCN/M/NiWO4 (30%) nanocomposite calcined at 450 and 550 °C for 3 h

Stability of a photocatalyst is particularly important for the widespread applications. To observe stability of the gCN/M/NiWO4 (30%) nanocomposite, the successive photocatalytic experiments were carried out. From Fig. 14a, it can be seen that RhB can be degraded up to 87.0% after four cycles, which suggests that the photocatalyst has a good photostability. Moreover, the XRD pattern of the gCN/M/NiWO4 (30%) nanocomposite after four cycles was provided, as presented in Fig. 14b. It can be clearly observed that the phase and structure of the nanocomposite remained unchanged, suggesting that the photocatalyst is stable after four runs. Also, the textural properties of the gCN/M/NiWO4 (30%) nanocomposite before and after photocatalysis were characterized, and the results are displayed in Fig. 14c. As can be seen, the specific surface area of the nanocomposite was decreased from 38.50 to 17.23 m2 g−1 after four runs. Decrease of the surface area could be ascribed to covering surface of the photocatalyst by intermediates of the degradation reaction, which block some parts of its surface.
Figure 14

a Reusability of the gCN/M/NiWO4 (30%) nanocomposite, b XRD patterns and c nitrogen adsorption–desorption data for the gCN/M/NiWO4 (30%) nanocomposite before and after photocatalysis

Generally, organic dyes from the wide variety groups are discharged into the environment. Hence, potential of the applied photocatalyst to remove these dye stuffs is urgently important. In this regard, RhB, MB, MO, and fuchsine (as colorful pollutants) and phenol (as colorless pollutant) were selected to measure photoactivity of the gCN/M/NiWO4 (30%) nanocomposite for degradations of these pollutants. As shown in Fig. 15, photocatalytic activity of the gCN/M/NiWO4 (30%) is about 12, 30, 52, 100, and 6 times greater than that of the pristine gCN in degradations of RhB, MB, MO, fuchsine, and phenol pollutants under the visible light, respectively. As a consequence, the fabricated hybrid photocatalyst has superior activity in removal of different pollutants, colorful and colorless, from aqueous solution.
Figure 15

Degradation rate constants of RhB, MB, MO, fuchsine, and phenol over the gCN, gCN/M, and gCN/M/NiWO4 (30%) samples under visible light

Conclusions

For summary, ternary gCN/M/NiWO4 nanocomposites with superior photocatalytic activity were prepared. The gCN/M/NiWO4 nanocomposite with 30% of NiWO4 showed the highest photocatalytic activity under visible-light illumination. The ternary photocatalyst showed the improved photocatalytic performance of 12, 30, 52, 100, and 6 times relative to the gCN and 11.8, 19, 21, 45, and 3.6 times relative to the gCN/M photocatalyst in degradations of RhB, MB, MO, fuchsine, and phenol, respectively. The magnetization data exhibited that the gCN/M/NiWO4 (30%) nanocomposite has saturation magnetization of 6.0 emu g−1, which is enough for magnetic recoverability of the photocatalyst from the treated system. Additionally, it was displayed that superoxide anion radicals are the main species in degradation of RhB by the gCN/M/NiWO4 (30%) photocatalyst. The results confirmed that heterojunction formation between gCN, Fe3O4, and NiWO4 increased the interfacial charge transfer and inhibited recombination of the e/h+ pairs, leading to increase of the produced reactive species to participate in the degradation reactions. In addition, the gCN/M/NiWO4 (30%) photocatalyst had also remarkable stability. This study demonstrated that the enhanced visible-light absorption, largely reduced recombination of the e/h+ pairs, increased surface area, ability to degrade different pollutants, and excellent stability ensure that the novel gCN/M/NiWO4 photocatalysts are promising photocatalysts for environmental remediation.

Notes

Acknowledgements

The financial support of this work by University of Mohaghegh Ardabili-Iran is acknowledged.

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Chemistry, Faculty of ScienceUniversity of Mohaghegh ArdabiliArdabilIran

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